Blown film production lines are typically limited in output by bubble stability. Blending Linear Low Density Polyethylene (LLDPE) with Low Density Polyethylene (LDPE) increases bubble stability, in part due to the higher melt strength of the LDPE. The increase in melt strength in part provides for an increase in film output. However, too high a melt strength can cause gels and poor quality film, as well as potentially limiting drawdown capabilities to thinner gauges. High melt strength resins also typically have reduced optics. Thus, there is a need for new compositions containing ethylene-based polymers, such as tubular LDPEs, that have an optimized balance of melt strength, optical and mechanical properties, for blown film applications.
The performance and application of films made from LLDPE-rich blends with LDPE will be strongly influenced by the rheology, density, and crystallinity of the selected LLDPE. The density and the crystallinity of the LLDPE can be varied over a wide range. The performance and application of films made from LDPE-rich blends with LLDPE will be increasingly influenced by the rheology and the molecular topology of the selected LDPE.
The processability of the blend will, to a large extent, be determined by the rheological properties of the LDPE blend components. In contrast to LLDPE, the density and crystallinity levels of the LDPE resin can only be varied in narrow ranges. Furthermore, these narrow ranges are to a large extent determined by the synthesis conditions needed to reach the desired molecular weight distribution (MWD) and the required rheological performance of the LDPE. For blends rich in LDPE, the density and the crystallinity level can be marginally influenced by the type of LLDPE and/or the allowed level of LLDPE. The level of LLDPE allowed will depend on the processing performance required, for instance bubble stability and bubble size Improved rheological properties of the LDPE blend component will lower the percentage of LDPE needed in the blend to reach a certain processing level. Furthermore, a lower percentage of LDPE means that the contribution to the film performance of the LLDPE blend component can be strengthened.
Improved rheological performance can be obtained by selecting LDPE resins with broad MWD and high melt strengths. Typically LDPE resins with very broad MWD are made using autoclave based reactor systems. The inherent residence time distribution in autoclave based reactor systems leads to broad MWD due to differentiated (time) growth paths of the polymer molecules. The high melt strength of the autoclave blend components is achieved by extremely broad, and, in some cases, bimodal MWD. The ultra high molecular weight fraction present in autoclave, high melt strength resins complicates blending at the molecular scale, could give rise to gel formation in the resultant films, while it negatively affects the optical performance of the films.
A tubular reactor operated under typical process conditions, operates at higher conversion levels and lower production costs, produces “less broad MWD” resins than typical broad MWD resins from an autoclave train, such as those used for extrusion coating or in blends. As a result, more of the “less broad MWD” tubular resin has to be blended into the LLDPE to reach a certain processing performance of the LDPE/LLDPE blends, or a lower melt index target has to be selected for the LDPE. The lower MI target will negatively affect the processability, such as increased processing pressures.
Thus, there remains a need for new LDPE-containing compositions, which can increase the melt strength and the processing performance of LDPE/LLDPE blends, and which can be made at low conversion costs in a tubular process. Furthermore, there is need for such compositions with improved performance in processing (maximum line speed and or large bubble operation) and/or film properties (mechanical and shrink performance and/or optical appearance). This requires LDPE resins made at lower melt index, high melt strength, and very broad MWD, but lacking the ultra high molecular weight fraction of “broad MWD” autoclave resins, and which can be made in a tubular process.
The uniform residence time in tubular reactors leads to narrower MWD, therefore very broad MWD can only be achieved in tubular reactors by applying extremely differentiated polymerization conditions, for example, as described in International Publication WO2013/078018, and/or application of a branching/cross-linking agent, for example, as described in U.S. Pat. No. 7,820,776. These tubular polyethylenes will have a specific composition (e.g. density) and functionality as determined by the applied process conditions, type and level of branching agent and/or comonomer. Undesirable gels in the polymer can be an issue, resulting from the use of branching or cross-linking agents.
Low density polyethylenes and blends are disclosed in the following: U.S. Publication 2014/0094583; U.S. Pat. Nos. 5,741,861; 7,741,415; 4,511,609; 4,705,829; U.S. Publication No. 2008/0038533; JP61-241339 (Abstract); JP2005-232227 (Abstract); and International Publication Nos. WO2010/144784, WO2011/019563, WO 2010/042390, WO 2010/144784, WO 2012/082393, WO 2006/049783, WO 2009/114661, U.S. 2008/0125553, EP0792318A1 and EP 2239283B1. However, such polymers do not provide an optimized balance of high melt strength and improved film mechanical properties, for blown film applications. Thus, as discussed above, there remains a need for new ethylene-based polymer compositions that have an optimized balance of melt strength, optics, processability and output, and toughness. These needs and others have been met by the following invention.